A tension leg platform (“TLP”) is a vertically moored floating structure used for offshore oil and gas production. The TLP is permanently moored by groups of tethers, called a tension leg, that eliminate virtually all vertical motion of the TLP. As a result of the minimal vertical motion of the TLP, the production wellhead may be located on deck instead of on the seafloor. The production wellhead connects to a subsea wellhead by one or more rigid risers.
The risers that connect the production wellhead to the subsea wellhead can be thousands of feet long and extremely heavy. To prevent the risers from buckling under their own weight or placing too much stress on the subsea wellhead, upward tension is applied, or the riser is lifted, to relieve a portion of the weight of the riser. The outermost riser, referred to herein as a casing, can be tensioned by hydraulic machines mounted to the TLP. An inner riser (e.g., a tie-back) is lifted, relative to the casing, to achieve a desired tension to relieve a portion of its weight from the subsea wellhead. However, the riser also needs to be shortened in length, relative to the casing, to compensate for the increase in length resulting from the increase in tension created by lifting the riser. Once the riser is shortened, the riser is then anchored to the production wellhead to maintain the desired tension.
In some solutions, the inner riser is shortened by clamping the riser while lifting under tension and removing an upper portion of the riser, for example by cutting. This solution is wasteful because material is removed from each successive riser after being lifted to a desired tension. In other solutions, the inner riser is shortened by tightening a threaded portion of the riser while lifting under tension. However, threading while under extreme axial loads is difficult. The threads bear the load of the riser while under tension and thus must be very robust and have very tight tolerances, both of which are very costly. Neither solution is desirable to shorten a riser after being lifted to achieve a desired tension.